1. Field of the Invention
This invention relates to fuses generally, and in particular it relates to fuses incorporating an indicating device.
2. Related Art
Fuses incorporating indicating devices are generally known in the art. An existing indicating fuse design 100 is illustrated in FIG. 3A. A fuse body 116 optionally includes an orientation ring 114 that may be disposed on the periphery of the fuse body 116. The orientation ring 114 serves to insure proper installation of the fuse 100. The precise structure and function of such an orientation device is described in U.S. Pat. No. 5,549,404, the disclosure of which is incorporated herein by reference.
Terminals 118 and 120 are disposed at opposite ends of the fuse. A main fuse element or link 122 is electrically connected to each of the terminals 118, 120. The majority of current passing through the fuse 100 is carried by the main fuse element 122 during normal operating conditions. "Normal operating conditions", or the normal operative state of the fuse is intended to mean a condition wherein the main fuse element 122 is intact, and current can pass from one terminal (118 or 120), through the fuse 100 without substantial interruption, and out of the fuse through the opposite terminal (118 or 120).
A conductor wire 142 is connected at one end to one terminal 118, and to an end of an indicator assembly 144. The indicator assembly includes an indicator pin 146, having a flat or "planished" end 148, a biasing member sleeve 156, and a biasing member 158 that contacts the bottom of the sleeve 156 and the head of the indicator pin 146 thereby tending to urge the indicator pin outward. The conductor wire 142 is attached to the planished end 148 of the indicator pin 146 thereby preventing the indicator pin 146 from being urged outwardly from the fuse by bias member 158 during normal operating conditions.
The fuse 100 normally includes a granular arc-quenching medium 150 in order to suppress and contain arcing during destruction of the main fuse link when the fuse 100 is "blown". The arc-quenching medium is typically a fine granular substance such as sand, terra alba, or mixtures thereof.
In certain lower amperage rated ruses, conductive wire 142 also serves as the main fuse link, thereby enabling omission of a separate main fuse link element 122 from the fuse 100 construction.
During normal operating conditions current is carried through the fuse primarily via main fuse link 122. In the event that the main fuse link 122 is destroyed (e.g., during an overload condition), current is diverted to the conductor wire 142. The capacity of conductor wire 142 to carry current is substantially less than that of main fuse link 122. Therefore, upon the above-mentioned diversion of current, conductor wire 142 is quickly destroyed. The conductor wire 142 is no longer capable of restraining the indicator pin 146, so bias member 158 urges the pin 146 outward from the fuse body 116 thereby indicating a blown fuse condition 112.
Various problems are associated with existing fuse constructions.
Historically, pin-indicator fuses generally require an arc-quenching filler in order to provide adequate performance for fuses rated at approximately 5 Amps or greater. This is particularly true for fuses having a current rating in excess of 15 Amps. However, inclusion of such a filler adds to the manufacturing costs of the fuse. Clearly, the filler adds to the materials cost of the fuse. Also, granular fillers are very destructive to the manufacturing equipment that is used to store, transport, and introduce the filler into the fuse. The filler also tends to adversely impact operation of the fuse indicator assembly by fouling the spring biasing element and otherwise inhibiting proper movement of the indicator assembly components. Moreover, the heat and electrical arcing associated with destruction of the fuse link can cause a reaction with the filler and produce aggregates or globules of filler which further act to prevent proper activation of the indicator assembly and movement of the indicator pin.
Another problem associated with existing designs is that the main fuse link may be deformed when the fuse is "blown" so that it comes into contact with the indicator pin thereby preventing the indicator pin from being pushed outwardly away from the fuse body to clearly indicate the fuse has been "blown".
A related problem is that the main fuse link can at least partially melt upon destruction and become affixed or welded to the conductor wire thereby preventing the indicator pin from properly emerging from the fuse.
Yet another problem associated with the existing designs is that gases produced during destruction of the fuse link can escape the fuse through the hole that the indicator pin passes through.
U.S. Pat. No. 4,023,133 to Knapp, Jr. discloses an indicating fuse having a fuse body containing an arc-quenching filler. A main fuse element connects two terminals. A wire is connected to one terminal and to a pin so as to restrain the pin during normal operation. When the wire melts, or is otherwise destroyed, the pin is urged outwardly and activates a complicated indicator assembly. A cap is provided which surrounds the biased pin actuator. As noted above, the inclusion of a granular filler adds to the cost of the fuse and may adversely affect the operation of the fuse. The indicator assembly is overly complicated and expensive. Moreover, there is no disclosure relating to locating the main fuse link so as to avoid interference with the conductor wire.
U.S. Pat. No. 4,204,182 to Knapp, Jr. discloses a fuse having a body containing an arc-quenching filler. A single fuse element is connected to a terminal at one end, and to a conductor rod at the other end. A partition wall is provided which partially surrounds the conductive rod. A tubular member is also provided at an end of the fuse which partially surrounds the conductive rod. The inclusion of a granular filler adds to the cost of the fuse and may adversely affect the operation of the fuse. The filler must be contained by a partition with a relatively large opening, and by a mass of silicone grease. In addition, there is no separate conductor wire to restrain the indicator pin.
U.S. Pat. No. 3,636,491 to Cameron discloses an indicating fuse that contains an arc-quenching filler. A plurality of fuse elements connect a pair of opposing terminals. A restraining wire is attached to an indicating plunger. A housing member surrounds the indicating plunger. The restraining wire passes through a large opening in the housing member, is connected to the indicating plunger, then exits the opening and is connected to a terminal. The inclusion of a granular filler adds to the cost of the fuse and may adversely affect the operation of the fuse. The indicator assembly appears especially prone to contamination by the filler due to the large opening in the housing member. There is no disclosure of locating the main fuse element(s) so as to minimize possible interference with the conductor wire.